Surgeons performing radial keratotomy incise by utilizing one of two techniques, a centripetal incising technique or a centrifugal incising technique. Each technique has its own advantages and disadvantages.
Centripetal incisions are sometimes preferred because, while using the same blade setting, the centripetal incisions are deeper than centrifugal incisions. Melles, G. R. and Binder, P. S. "Effects of Radial Keratotomy Incision Direction on Wound Depth", Refract Corneal Surg, 6:394, 1990. The differences in incisional depth are due to the uphill vector forces associated with centripetal incisions. Centrifugal incisions are more shallow because of downhill vector forces directed out of the incision.
Centripetal incisions are retraceable and more uniform than centrifugal incisions. Most importantly, the centripetal incisions square off at the optical zone, thus forming a perpendicular architecture at the stopping point of the incision. Centrifugal incisions, on the other hand, round off at the optical zone. The physical characteristics of the cornea such as corneal compressibility, flexibility and intraocular pressure differ with the incisional depth and angle of the two incisional techniques.
Disadvantages of using the centripetal technique are known. A risk of extending centripetal incisions beyond the optical zone exists. Overextending the incision into the optical zone may overcorrect, as well as, reduce vision or cause glare. Such overextension resulting in these injuries may require a corneal transplant. Another disadvantage of the centripetal incision is difficulty in incising straight. Centrifugal incisions, however, are straighter than centripetal incisions.
A blade has been developed, i.e., System DuoTrak, for both centripetal and centrifugal incisions. Casebeer, J. C. and Shapiro, D. R. "Blade designed for improved safety and accuracy in radial keratotomy", J Cataract Refract Surg, 19:314, 1993. In the System DuoTrak, a front diamond blade first incises centrifugally with a full cutting edge. Then a centripetal incision retraces the centrifugal incision by employing a 200 u reverse straight cutting edge at its deepest aspect and a superficial blunt edge at the shallow aspect of the blade. The centrifugal incision is shallow, rounded off at the optical zone, and of irregular depth. The centripetal incision retraces the centrifugal incision correcting these deficits. The dull area of the reverse cutting edge guides the blade back over the centrifugal incision, preventing deviation from the centrifugal incision. The deeper aspect of the reverse cutting edge deeply and uniformly recuts the bottom of the centrifugal incision. When the reverse cutting edge reaches the optical zone, the deep reverse cutting edge stops because of the dull superficial edge of the blade. At this point, the reverse cutting edge squares off the optical zone. The reverse cutting edge will not incise past the optical zone because of the dull aspect of the blade.
Updegraff et al. have described the DuoTrak histology and optical zone structure. Updegraff, S. A., McDonald, M. B. and Benerman, R., "Freeze Fracture Analysis of American, Russian and DuoTrak Incisions", ARVO Abstract, Invest Opthalmol Vis Sci 34 (Suppl): 801, 1993. Updegraff et al. discovered that the centripetal incision made by the reverse cutting edge is irregularly displaced in relation to the centrifugal incision made by the diamond front cutting edge. The centripetal incision irregularly tears the collagen at the base of the first pass incision. Furthermore, the reverse cutting edge does not square off the optical zone. Instead, a portion of midstroma is unincised. Although ramifications of these problems are yet unknown, Updegraff et al. believe they do not duplicate the ideal configuration of the centripetal incision. Furthermore, the DuoTrak incisions may well be less repeatable than centripetal incisions.